The present invention relates to a biaxially oriented laminated polyester film for magnetic recording media and, more particularly, it relates to a biaxially oriented laminated polyester film for magnetic recording media, which has excellent surface uniformity, slittability, running property and cost performance, and in which a magnetic tape produced therefrom shows a very small number of scratches and a very small amount of abrasion dust.
Biaxially oriented polyester films are widely used as industrial materials. With the recent demand for higher-grade polyesters, especially, for magnetic recording media, they are strongly required to have uniform surfaces. The abrasion property of polyester films cannot be said satisfactory, which is are represented by a scraped surface layer due to the wear of a film or drop-off of the particles from the surface. For example, when such a polyester film is used as a base film of a video tape, the tape is apt to meander when running. The improvement of these properties is strongly demanded and, especially, polyester films for high-density magnetic recording media are required to fully satisfy these demands.
As a means for improving the abrasion resistance and the running property of a polyester film, there has been known a method of making the film surface appropriately rough by mixing inert fine particles with the film. By this method, these properties are improved to some extent but cannot always be said to be satisfactory.
For example, in case of using as the fine particles, the so-called deposited particles which are produced from the residue of a catalyst after the production of a polyester, the fine particles are easily broken when the film is stretched, so that the abrasion resistance and the running property of the film are insufficient and the reuse of the deposited particles is difficult.
In case of adding inorganic compound particles inert to polyesters, such as silicon oxide, barium sulfate, titanium dioxide and calcium phosphate to a polyester film, these particles are not broken or deformed when the film is stretched, and endow comparatively sharp protuberances for the film surface, thereby improving the running property of the film. However, since the particle size distribution of these particles is generally wide and the particles easily drop from the film surface, when the film is used for a magnetic recording medium, they often cause the deterioration of the electromagnetic conversion property (signal recording property) and a large number of dropouts.
To solve these problems, a method of using inorganic or organic particles having a sharp particle size distribution has been proposed. For example, Japanese Patent Application Laid-Open (KOKAI) No. 62-207356 (1987) discloses a method of using monodisperse silicon oxide particles, and Japanese Patent Application Laid-Open (KOKAI) No. 59-217755 (1984) discloses a method of using organic crosslinking particles obtained by emulsion polymerization. In the case of using silicon oxide particles, however, since the hardness of the particles is high, a silicon particles-containing film is apt to damage the base material which is in contact with the film, and the improvement of the abrasion resistance is insufficient. On the other hand, in the case of using organic crosslinking particles, not only the heat resistance is insufficient but also the particles are easily deformed when the film is stretched. In addition, since monodisperse particles are comparatively expensive, the film containing a large amount of monodisperse particles is very disadvantageous in the point of view of cost.
As a way for solving the problem in the abrasion resistance, a method of adding particles having a high Mohs hardness such as aluminum oxide particles to a base film has been proposed (e.g., Japanese Patent Application Laid-Open (KOKAI) No. 1-306220 (1989). In this method, however, it is necessary to add a large amount of aluminum oxide particles to a base film in order to sufficiently prevent the production of scratches or abrasion dust on the magnetic tape, resulting in the following problems.
(1) Since aluminum oxide particles are comparatively expensive, the film containing a large amount of monodisperse particles is very disadvantageous from the view-point of cost.
(2) In the film formation process and in the magnetic tape production process, the cutting blade is so greatly damaged at the slitting step that frequent change of cutting blades is necessary, thereby increasing the manufacturing cost and reducing the productivity. If the cutting blades are not changed frequently at the slitting step when a magnetic tape is produced, the slitting property of the magnetic tape is deteriorated. Such deterioration of the slitting property causes that the cut face is sometimes turned up in the shape of streaks and, in some cases, cuttings are drop off the cut face. Furthermore, abrasion dust may drop off the magnetic layer, thereby inducing dropouts.
The following polyester films have been also proposed.
Japanese Patent Application Laid-Open (KOKAI) No. 3-1941 (1991) proposes a laminated polyester film having at least one outermost layer composed of a polyester layer containing 0.01 to 5 wt % of inorganic particles having an average particle diameter of not more than 5 .mu.m and a Mohs hardness of not less than 8.
Japanese Patent Application Laid-Open (KOKAI) No. 4-65239 (1992) proposes a biaxially oriented laminated polyester film comprising a base film and outer layers disposed on both surfaces of the film, wherein at least one outermost layer contains 0.01 to 2.0 wt % of inorganic particles having a Mohs hardness of 2 to 5, an average particle diameter (d.sub.A) of 0.1 to 2.0 .mu.m, and the thickness (t.sub.1) of the outermost layer satisfies the following expression. EQU 0.20 d.sub.A .ltoreq.t.sub.1 .ltoreq.5.0 d.sub.A
Japanese Patent Application Laid-Open (KOKAI) No. 4-105935 (1992) proposes a biaxially oriented laminated thermoplastic resin film comprising a film (layer B) containing a thermoplastic resin B as the main ingredient, and a film (layer A) disposed on at least one side of the layer B and containing a thermoplastic resin A as the main ingredient, wherein the thickness of the layer A is 0.01 to 3 .mu.m, and the thermoplastic resin A contains 0.1 to 20 wt % of spherical calcium carbonate particles having an average particle diameter of 0.2 to 5 times as large as the thickness of the layer A.
However, there is no end to the demand for improved polyester films, and a biaxially oriented laminated film which shows a very small number of scratches and a very small amount of abrasion dust, which has a more excellent running property when manufactured into a magnetic tape, and which has improved slittability at the time of film formation is demanded.
As a result of various studies undertaken by the present inventors so as to solve the above-described problems, it has been found that by laminating a polyester layer (B) on a polyester layer (A) containing inorganic particles (a) having a Mohs hardness of not less than 7 and an average particle diameter of 0.005 to 0.5 .mu.m, and inorganic particles (b) having a Mohs hardness of less than 7, a particle-size distribution of not more than 2.00, an aspect ratio (major axial diameter/minor axial diameter) of 1.0 to 2.0, a specific surface area of 1.6 to 40 m.sup.2 /g, an average particle diameter of 0.05 to 1.0 .mu.m, the product of the specific surface area and the average particle diameter of 2.4 to 7.0, wherein the content of the inorganic particles (a) is 0.15 to 1.0 wt % based on the polyester content in the layer (A) and not more than 0.10 wt % based on the total polyester content in the layers (A) and (B), and the content of the inorganic particles (b) is 0.10 to 2.0 wt % based on the polyester content in the layer (A) and not more than 0.4 wt % based on the total polyester content in the layers (A) and (B), the obtained biaxially oriented laminated film having a three-dimensional center-line average roughness of the layer (A) of 0.005 to 0.045 .mu.m, the quotient of the three-dimensional center-line average roughness of the layer (A) thereof by the three-dimensional 10-point average roughness of the surface of the layer (A) of 4 to 13, a three-dimensional center-line average roughness of the layer (B) of 0.002 to 0.020 .mu.m, the quotient of the three-dimensional center-line average roughness of the layer (A) by the three-dimensional center-line average roughness of the layer (B) of 1.1 to 4.0, and an improved slittability at the time of film formation, and showing a very small number of scratches, a very small amount of abrasion dust and having a more excellent running property than that of a conventional one when manufactured into a magnetic tape. On the basis of this finding, the present invention has been achieved.